1. Field of the Invention
The present invention relates to a holographic optical element; more particularly, a dual-wavelength polarization-selective holographic beam splitter useful in an optical head.
2. Description of the Related Art
Optical heads are utilized to guide radiation, e.g., light, emitted from a laser to the surface of a data storage medium and to guide the light reflected from the data storage medium to a detector. It is desirable to have the incident light traveling in a direction normal to the surface of the data storage medium, and thus the reflected beam travels the same path as the incident beam. Accordingly, it is necessary to separate the incident and reflected beams so that the reflected beam can be directed to a detector. This process of imaging light onto the surface of a recording medium and detecting the reflected light, for optically storing and retrieving data or for tracking in an optical, magnetic or magneto-optic storage system, requires a large number of expensive optical elements.
In the following discussion of conventional optical heads and existing designs of holographic beam splitters, an optical head for use, e.g., in a compact disk player, is described to facilitate the description of the related art. A first conventional optical head will be described with reference to FIG. 1. The optical head of FIG. 1 includes a laser 10 which provides a beam of incident light 12. The light emitted by laser 12 is polarized and the laser is rotated to select the direction of polarization with respect to the direction of the incident beam 12. The incident beam 12 can have either P or S polarization--in general the laser is rotated so that beam 12 is P-polarized. Polarized beam 12a passes through collimator 14 and then enters polarization beam splitter 18 which is designed to allow a first polarization-type incident beam (i.e., P-polarized beam 12a) to pass therethrough without change. The polarized incident light beam then enters a quarter-wave plate 20 which converts the P-polarized incident beam 12a to circularly polarized incident beam 12b. The circularly polarized incident beam 12b can be right or left circularly polarized depending on the orientation of the quarter-wave plate 20 and the polarization, i.e., S or P polarization, of the polarized incident beam 12a. Beam 12b is focussed on the surface of an optical storage medium, for example, a compact disk 22, by an objective 24.
Upon reflection from the surface of compact disk 22, circularly polarized incident beam 12b is converted to a reflected beam having the opposite type circular polarization, a right-hand circularly polarized beam 12b is converted to a left-hand circularly polarized beam and vice versa. Thus, circularly polarized reflected beam 12c has the opposite circular polarization type from circularly polarized incident beam 12b. Beam 12c is re-collimated by objective 24, and then converted to second polarization type reflected light 12d (i.e., S type polarization) having an opposite polarization type from first polarization-type incident light 12a. Polarization beam splitter 18 causes beam 12d to be directed towards a detector 26. A concentrating lens 28 is provided between polarization beam splitter 18 and detector 26 to focus beam 12d on the detector 26.
Conventional polarization beam splitter 18 provides a 90.degree. re-direction of the reflected beam 12d. Accordingly, detector 26 must be placed alongside the polarization beam splitter 18 so that beam 12d is incident on detector 26. When the optical head is moved to track the beam over the surface of the storage medium 22 the space occupied by detector 26 becomes significant. Further, concentrating lens 28 must be carefully aligned with polarization beam splitter 18 (or the optical path or redirected beam 12d) and detector 26 must be aligned with the optical path to allow detector 26 to focus and track beam 12b on disk 22, and to read data on disk 22. This alignment must be accurate to within tens of microns. In addition, concentrating lens 28 is an expensive element, due in part to the need for astigmatic power to provide proper focusing.
A second conventional optical head, manufactured by Pencom International Corporation, employs a hologram as a beam splitter. The intensity of light provided to the detector after reflection from a nearly 100% reflector using the Pencom design has been reported to be less than 10% of the light emitted by the laser. This extreme drop in intensity places severe demands on the amplifying circuitry used to control the focusing and tracking servos and requires an optical medium with high reflectivity. Moreover, the Pencom design allows a significant amount of reflected light to enter the optical cavity, which may result in instability of the laser. The Pencom holographic optical head will be described with reference to FIGS. 2A-B.
In the Pencom holographic optical head, laser 10 provides an incident light beam 30 which passes through collimator 14 and then into a holographic optical element 32. Holographic optical element 32 is not polarization-selective, and a portion of all incident light 30a, regardless of polarization, is diffracted. Approximately 10% of the incident beam 30a is diffracted into each of the plus and minus first order diffractions, and some, approximately 1-5%, of the incident beam 30a is diffracted into each of the plus and minus second and higher order diffractions. Smaller percentages of the incident beam 30a are diffracted into higher order diffractions. As a result, holographic beam splitter 32 has a forward efficiency of approximately 70%. Only the undiffracted incident beam 30b is focused on optical storage medium 22 by objective 24.
A reflected beam 30c is re-collimated by objective 24. When reflected beam 30c enters holographic beam splitter 32 the reflected beam 30c is again diffracted by holographic beam splitter 32. In the Pencom design, detector 26 is placed so that it receives diffracted reflected beam 30d, comprising the portion of the reflected beam 30c which is diffracted into the plus or minus first order after beam 30d again passes through collimator 14. Collimator 14 takes the place of concentrating lens 28 in the conventional design of FIG. 1.
The holographic optical element 32 of the Pencom optical head is shown in FIG. 2B. First and second substrates 34a, b are provided on each side of a holographic layer 36. Holographic layer 36 has a plurality of fringes 38 having a spacing d. In the holographic optical element 32 used in the Pencom optical head, the spacing d of fringes 38 is relatively large, on the order of 4 .mu.m.
The downfall in the Pencom design is that only 10% of the reflected beam 30b is diffracted into the first order. Thus, the diffracted reflected beam 30d has an intensity of only 10% of reflected beam 30c. Assuming 100% efficiency of each of the elements and 100% reflection from the surface of the storage medium 22, the intensity of diffracted reflected beam 30c is only 7% of the intensity of the incident beam 30a, i.e., 10% of the 70% of the incident beam which is not diffracted by holographic optical element 32.
In addition, since the holographic optical element 32 utilized in the Pencom design is not polarization selective, the undiffracted reflected beam 30e passes back into the optical cavity of laser 10 which may cause instability of the laser. Moreover, with the holographic beam splitter 32 utilized by Pencom, it is not possible to remove the high order diffracted beams, i.e., diffracted beams above the first order.
NEC Corporation has constructed a holographic optical element to be used in a third conventional optical head, as described in "Compact Optical Head Using A Holographic Optical Element For CD Players," Kimira et al., Applied Optics, Vol. 27, No. 4, pp. 668-671. In the NEC design, multiple holograms containing tracking and focusing information used to track and focus the reflected beam on the detector array used for sensing the data are stored in the hologram of the holographic optical element. The surface relief holograms employed in the NEC design are not polarization selective.
Another holographic optical head is disclosed in U.S. Pat. No. 4,497,534 to Sincerbox. The Sincerbox holographic optical head, however, requires all of the optical elements in the head to be maintained in an environment having essentially the same index of refraction as the materials forming the optical elements to prevent total internal reflection of the diffracted beams within the holograms.
The polarization selectivity of holograms was discussed in the Appendix of "Coupled Wave Theory For Thick Hologram Gratings," H. Kogelnik, Bell System Technical Journal, Vol. 48, No. 9, November 1969, p. 2909. One phenomenon noted by Kogelnik is that holograms which diffract light perpendicularly, so that the diffracted light travels in a direction perpendicular to the direction of the incident light, exhibit the greatest polarization selectivity.
It is known that a hologram will diffract the "S" polarized portion of incident light by 90.degree. while allowing the "P" portion of the polarized light to pass without diffraction. However, as known to those of ordinary skill in the art, the "S" polarized light cannot escape from the holographic material or the substrate on which the holographic material is supported since the light will be totally internally reflected, regardless of the angle that the incident beam makes with the surface of the hologram, after the light is diffracted by 90.degree.. Thus, although it has been possible to create a hologram which selects one type of polarized light, it has been necessary to either use a prism to extract the diffracted light from the hologram or the supporting substrate or place the hologram in an environment having an index of refraction essentially the same as that of the hologram One method of providing an environment having essentially the same index of refraction as the hologram, utilized by Sincerbox, is to have the substrate on which the hologram is supported (plural elements laminated with optical adhesive) extend between all of the elements in the optical head.
Optical heads used in, for example, compact disk players, operate in conjunction with a diode laser. However, there are no known holographic recording materials which can be utilized to record holograms having a large diffraction efficiency (i.e., near 100%) when the wavelength of the light used to record (or create) the hologram is in the infrared range--diode lasers emit light in the infrared range. Accordingly, it has been difficult to create holograms, and thus holographic optical elements, for use with diode lasers.
In order to create holograms which reconstruct infrared light with high efficiency, a multi-step process has been used to record the holograms. One such process is disclosed in "Efficient and Aberration-Free Wavefront Reconstruction From Holograms Illuminated At Wavelengths Differing From The Former Wavelength," L. H. Lin, et al., Applied Optics, Vol. 10, No. 6, June 1971, pg. 1314-1318. This multi-step method involves recording a first hologram at the infrared wavelength, illuminating the first hologram using blue light, and using the blue light diffracted by the first hologram to create a second high-efficiency hologram useful with infrared light.